Hip-joint prosthesis with a shaft to be inserted into the femur

ABSTRACT

A hip-joint prosthesis includes a shaft which is configured to be inserted into the femur and whose surface has an osteoinductive finish. This finish is provided exclusively in the metaphyseal portion of the shaft and laterally from the line delineating the maximum antero-posterior dimension of the shaft cross section. This ensures a better involvement of the metaphyseal spongiosa in the flow of forces, without compromising the ability to perform follow-up surgery on the prosthesis.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/659,346, filed Feb. 5, 2007, which is a national stage applicationunder 35 USC 371 of International Application No. PCT/EP2005/008513,filed Aug. 5, 2005, which claims priority from European Application No.04 018 714.8, filed Aug. 6, 2004, the entire contents of which areincorporated herein by reference.

FIELD AND BACKGROUND OF THE INVENTION

The spongy bone tissue in the metaphysis of the femur includes acomplicated structure of bone trabeculae via which the parts of the bonesubjected to compression loads and tensile loads at the femoral neck,the greater trochanter, the lesser trochanter and the diaphysis areconnected in a manner transmitting compression and tension. In theirtotality, they form continuous tension and compression trajectories(Farbatlanten der Medizin [Colour Atlas of Medicine], Volume 7:Locomotor apparatus I., published by Thieme Verlag, Stuttgart, 1992).When the shaft of a hip-joint prosthesis is inserted, the primarytension trajectories in particular, which connect the femoral neck tothe opposite intertrochanteric surface area of the bone, are for themost part interrupted. When they subsequently are no longer involved inthe transmission of forces, they regress. This applies in particularwhen using prostheses whose prosthesis shaft is clamped in the diaphysisand in which the proximal, metaphyseal area of the femur, especially inits lateral part, is barely involved in force transmission. Attemptshave been made, using what are called tension anchors, to connect theprosthesis shaft to the area of the greater trochanter and in this wayto involve the latter in the flow of forces. A rod connected to theprosthesis shaft was guided through the greater trochanter and providedon the outside with a locking nut so that, upon loading of the hipprosthesis, a tension is exerted on the greater trochanter (U.S. Pat.No. 3,995,323, EP-B-93230, DE-B-1943598). However, it has been foundthat, because of the constant alternating loading, mechanical tensionanchors of this kind quickly come loose and therefore are effective onlyfor a short time. It is also known to design the shaft, or a wingprojecting laterally therefrom into the area of the greater trochanter,in such a way that an intimate connection is obtained with the bonesubstance growing into pores or openings of this wing (GB-A-1030145,FR-A-2356465, EP-A-128036, EP-A-222236, EP-A-95440, EP-B-601223,EP-A-1044665, U.S. Pat. Nos. 5,755,811, 4,718,915, 5,370,698,FR-C-2194123). To promote connection of the bone to the prosthesissurface, it is also known to make the prosthesis surfaceosteoconductive. This term denotes surfaces which tolerate adjacent bonegrowth. These include surfaces made of titanium alloys and coatingswhich contain calcium phosphate or hydroxyapatite (EP-A-761182, WO9308771).

More recently, substances have been made known which not only toleratebone growth like the osteoconductive surfaces, but stimulateundifferentiated pluripotent stem cells for conversion to bone cells(Albrechtsson, Johansson: Osteoinduction, Osteoconduction andOsseointegration, in: Gunzburg Press: The use of bone substitutes inspine surgery; Springer. Denissen, H. et al.: Ceramic hydroxyapatiteimplants for the release of bisphosphonate, in: Bone and Mineral 1994,pages 123-134. Yoshinari, M. et al.: Bone response to calciumphosphate-coated and bisphosphonate-immobilized titanium implants, in:Biomaterials 2002, pages 2879-2885. Yoshinari, M. et al.: Immobilizationof bisphosphonates on surface-modified titanium, in: Biomaterials 2001,pages 709-715). These substances include bisphosphonates and bonemorphogenic proteins (BMP). These can also be used to finish thesurfaces of bone prostheses, including hip prostheses(US-A-2002/0049497, US-A-2002/0127261). They lead to a very intimateconnection of the prosthesis surface with the bone, which may beundesirable in the event of follow-up surgery because removal of theprosthesis from the bone may be impeded by this. This applies inparticular when the shaft of a hip prosthesis is equipped in itsentirety or to a substantial extent with such a substance (EP-A-478532,U.S. Pat. No. 6,296,667, DE-A-19508753).

SUMMARY OF THE INVENTION

The object of the invention is to improve the fixing of a femoral hipprosthesis in the bone without compromising the ability to performfollow-up surgery. The solution according to the invention lies in thefeatures of the invention as disclosed herein.

Accordingly, in a hip joint prosthesis with a shaft to be inserted intothe femur, the osteoinductive coating is provided exclusively in a partof the shaft situated laterally from the line of maximum AP(antero-posterior) dimension in the metaphyseal portion.

In an earlier patent application (PCT/EP03/05292) not belonging to thepublished prior art, it was proposed that the finish with theosteoinductive substance be provided exclusively in the trochantericarea, which is defined as follows. Starting from the point ofintersection between the centre line of the femoral neck and the centreline of the proximal end of the diaphysis, the trochanteric area lieslaterally from the tangent drawn from this point of intersection to thetop edge of the head of the hip, and laterally from the part of thecentre line of the femoral neck that continues this tangent.

According to the present invention, the demarcation of the surface areato be finished with the osteoinductive substance is differently defined.The definition according to the invention is that the osteoinductivefinish of the prosthesis surface is provided in that part of themetaphyseal portion in which there are particularly many surface areaswhose directional normal contains a lateral component. Earlierexperience has shown that, in these surface areas, there can be noexpectation of a connection to the bone permitting transmission oftensile forces. By virtue of the osteoinductive coating, however, such aconnection is made possible for the most part. The earlier applicationproposes the opposite, namely arranging the coating on surface areaswhich are undercut in relation to the lateral direction, i.e. pointingin the medial direction.

It is particularly expedient for the substance to be incorporated into acoating, which is also intended to be porous. The coating can be of anydesired type. For example, it can be a porous metal layer. Coatings ofparticular advantage are ones which are originally osteoconductive and,for example, consist of calcium phosphate or hydroxyapatite.

The effect of the invention is that, very quickly after the operation,bone cells develop in immediate proximity to and in connection to theprosthesis surface. The result of this is that relative movementsbetween the bone surface and the bone do not initially cause formationof a gap or an intermediate connective tissue layer which makessubsequent intimate connection more difficult or impossible. By virtueof the invention, there is a more rapid accumulation of bone on thetrochanteric surface of the prosthesis and more rapid incorporation ofbone into the pores and recesses thereof, so that the trochanteric areaof the bone quickly achieves a permanent connection to the prosthesisand, as a result of this, is involved in the transmission of forces. Bycontrast, on the other surface areas, the connection to the bone isachieved only to the previously expected extent. Outside thetrochanteric area, which is readily accessible in the event of follow-upsurgery and therefore does not cause any problems even when the bone isvery strongly bound, the physician, when performing follow-up surgery,therefore finds exactly those conditions with which he is alreadyaccustomed.

The part of the surface containing the osteoinductive substanceexpediently comprises pores or undercuts in relation to the lateraldirection, so that the bone substance formed as a result of theosteoinduction can not only adhere to the surface but can also anchoronto it with a form fit.

In the middle of the trochanteric part of the bone, the spongy substanceis sometimes less dense than it is near the cortex. For this reason,those portions of the osteoinductively finished surface areas of theprosthesis that point in the ventral and dorsal directions arepreferably situated at a certain distance from the mid-plane of the boneand nearer the cortex. Therefore, the part of the prosthesis formingthese surface areas ought not to be too thin in the AP direction. Itsthickness, and thus the distance between said dorsal and ventral surfaceareas, is expediently over 6 mm, and more advantageously over 9 mm toapproximately 15 mm.

The growth of fresh bone cells onto the prosthesis surface can bepromoted by a press-fit of the surface areas in question. It istherefore expedient if the surfaces in question, and their matingsurfaces, are made wedge-shaped in the direction in which the prosthesisis inserted into the bone, and if the rasp assigned to the prosthesis,and used to shape the receiving space for the prosthesis shaft, isprovided with slightly smaller cross-sectional dimensions so that, whenthe prosthesis shaft is pushed into the space formed by the rasp, thesurface areas in question displace bone substance.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below with reference to thedrawing, in which:

FIG. 1 shows a ventral view of a femoral hip-joint prosthesis,

FIG. 2 shows a cross section along line II-II in FIG. 1,

FIGS. 3 and 4 show alternative cross-sectional shapes,

FIG. 5 shows a cross section along line V-V in FIG. 1.

FIG. 6 shows a cross section along line II-II in FIG. 1 with projection9 detached from shaft 3.

FIG. 7 shows a cross section of a rasp for shaping the receiving spacefor the shaft of the hip-joint prosthesis.

DETAILED DESCRIPTION OF THE INVENTION

The hip prosthesis according to FIG. 1 comprises a joint head 1, and aneck 2 which has a shaft 3. The latter has a diaphyseal portion 4 to beanchored in the diaphysis of the bone, and a metaphyseal portion 5 to beanchored in the metaphysis of the bone. The diaphyseal portion isdimensioned such that it ensures primary anchoring of the shaft in thediaphysis of the femur. The person skilled in the art will see, fromlooking at FIG. 1, how the prosthesis will lie in the bone, and he orshe will therefore also know where the border 8 between the diaphysealand metaphyseal portions of the prosthesis lies.

Whereas the diaphyseal portion 4 of the shaft can bear on the strongcortical bone of the diaphysis and effect primary anchoring there, themetaphyseal portion lies mainly in the spongy bone tissue of themetaphysis.

FIG. 2 shows a location 7 having the greatest dimension of the shaft 3in the AP direction (antero-posterior). The connection of the locationsof maximum AP dimension is shown in FIG. 2 as line 7. This extends theentire length of the prosthesis. For the invention, only its extent inthe metaphyseal portion 5 is of interest. Laterally from this line, thesurface of the shaft in the illustrative embodiment in FIG. 3 iscomposed exclusively of surface areas in which the surface normal has acomponent oriented in the lateral direction. This means that it is notpossible, by conventional means, to obtain a connection between thesesurfaces and the bone which is able to take up medially directedprosthesis forces. These forces in fact presuppose a tensile connectionbetween these surface areas and the bone. In the illustrativeembodiments according to FIG. 4 or FIG. 5, undercut surface areas arealso present laterally from the location or line 7, in which undercutsurface areas the surface normal has no component oriented in thelateral direction. However, the laterally directed surface portions alsopredominate here.

According to the invention, a part 6 of the shaft surface lying in themetaphyseal portion (that is to say above the line 8) and laterally fromthe line 7 is finished with an osteoinductive substance. This is shownby stippling in the drawing. The finish is preferably contained in alayer of osteoconductive material such as hydroxyapatite. By means ofthis finish, it is possible to strengthen the connection between thesurface areas in question and the bone tissue in such a way that it isable to transmit tensile forces. This means that the tensiontrajectories in the spongy bone tissue are involved in the take-up offorces and are accordingly not broken down.

In the illustrative embodiment in FIG. 1, the area finished according tothe invention includes a projection 9 which lies in the trochantericarea of the metaphysis. Such trochanteric projections are customary forimproving the anchoring of the shaft in the bone tissue and forpreventing the shaft from turning relative to the bone.

A surface configuration promoting the connection to the bone can also beprovided in the other areas of the shaft 3, i.e. medially from the line7 and distally from the line 8, for example a coating withhydroxyapatite or calcium phosphate. However, it should contain noosteoinductive components there, because otherwise the removal of theprosthesis shaft from the bone in the event of follow-up surgery is madevery difficult.

FIG. 2 shows that the projection 13 has a considerable thickness in theantero-posterior direction. Its anterior and posterior surface areas 6are therefore set back from the central area in which the spongy bonesubstance is in many cases depleted, and they are situated in a morecompact area closer to the cortical bone. Hence, the likelihood of agood connection between the bone surface and the bone substance isfurther enhanced.

FIG. 5 shows the cross-sectional shape of the projection 9 in sectionaldirection V-V, which also corresponds approximately to the direction ofinsertion. If the hollow space, which has been prepared by means of arasp (FIG. 7) in order to receive the prosthesis, is slightly smallerthan the prosthesis shape, the insertion of this wedge shape into thebone causes a displacement of bone substance and, as a result, anincrease in the pressure exerted on the prosthesis surface by the bonesubstance. In this way too, a rapidly growing and intimate union of theprosthesis surface with the bone is promoted.

To ensure that the intimate connection between the surface of thetrochanteric projection and the bone is not problematic in the event offollow-up surgery, the projection 9 can be detachable from the shaft 3(FIG. 6). For example, it can be connected to the shaft 3 by means ofscrews or other connecting means and can be detached from said shaft 3before the shaft is removed from the bone. The projection can then bemore easily released from the bone surrounding it and connected to it.

The surface finish according to the invention is not limited to theprosthesis shape shown in FIGS. 1 and 2. Examples of othercross-sectional shapes are shown in FIGS. 3 and 4. Reference number 7there designates the cross-sectional point of maximum thickness in theAP direction, laterally from which the invention permits anosteoinductive finish of the prosthesis surface.

The invention claimed is:
 1. A hip-joint prosthesis including a shaftwhich is configured to be inserted into a femur and whose surface isfinished with an osteoinductive substance that is provided exclusivelyon a part of the shaft situated on a lateral side of a most lateralboundary delineating a maximum antero-posterior dimension of the shaftcross section in a metaphyseal portion of the shaft and whose surface isfree from an osteoinductive substance on a medial side of the boundary,wherein the osteoinductive substance is formed by a coating on the partof the shaft or is contained in a coating on the part of the shaft. 2.The hip-joint prosthesis according to claim 1, wherein theosteoinductive substance comprises a bisphosphonate or a bonemorphogenic protein.
 3. The hip joint prosthesis according to claim 1,wherein at least the part of the prosthesis surface containing theosteoinductive substance is porous.
 4. The hip-joint prosthesisaccording to claim 1, wherein the part of the prosthesis finished withthe osteoinductive substance comprises at least two opposite surfaceswhich point in the ventral and dorsal directions and enclose a spacingin the anteroposterior direction of more than 6 mm.
 5. The hip-jointprosthesis according to claim 1, wherein the part of the prosthesisfinished with the osteoinductive substance is formed at least partiallyby a projection extending from the shaft.
 6. The hip-joint prosthesisaccording to claim 5, wherein the projection is configured to bedetachable from the shaft.
 7. The hip-joint prosthesis according toclaim 1, wherein the part of the prosthesis finished with theosteoinductive substance comprises at least two opposite surfaces whichpoint in the ventral and dorsal directions and enclose a spacing in theanteroposterior direction of more than 9 mm.
 8. The hip-joint prosthesisaccording to claim 2, wherein the part of the prosthesis finished withthe osteoinductive substance is formed at least partially by aprojection extending from the shaft.
 9. The hip joint prosthesisaccording to claim 3, wherein the part of the prosthesis finished withthe osteoinductive substance is formed at least partially by aprojection extending from the shaft.
 10. The hip-joint prosthesisaccording to claim 4, wherein the part of the prosthesis finished withthe osteoinductive substance is formed at least partially by aprojection extending from the shaft.